Many types of valves exist for industrial application including ball valves, globe valves, butterfly valves, plug valves and so forth but a type of valve that has been universally popular, particularly for heavy duty industrial and high pressure applications in oil and gas fields, chemical plants, refineries, and so forth is known generally as a "gate valve". Gate valves traditionally function by moving a gate that is slidably positioned between an upstream and a downstream valve seat within a valve body, the gate having in one position an opening through it that, when aligned with a flow passageway through the valve body, permits fluid flow and the gate having in another portion a solid surface that prohibits fluid flow. Thus, it is called a gate valve since the gate can be opened or closed to allow flow or prohibit flow.
A basic gate valve employs a gate member that is of uniform thickness and slides between an inlet and an outlet valve seat. As long as a close tolerance is maintained in the spacing between the valve seats and the thickness of the gate then functions satisfactorily. As wear occurs, leakage through the gate valve can take place. A more refined gate valve is a type that compensates for wear and manufacturing tolerances and has provision for controlling the width or thickness of the gate as it seals between an opposed inlet passageway surrounded by an inlet valve seat and an outlet passageway surrounded by an outlet valve seat. That is, improved gate valves are designed so that in the closed position the thickness of the gate is automatically increased to more effectively seal the passageway through the valve body.
One type of improved gate valve has a gate that is moved vertically between open and closed positions, the gate being between opposed inlet and outlet seals surrounding inlet and outlet passageways. The gate is bifurcated having a first portion with a planar outer surface and a concave inner surface. The other half of the bifurcated gate has a planar outer and a convex inner surface. The second half of the gate is free to displace within limits as confined by the concave/convex interrelationship. This type of valve expands in both the fully open and fully closed positions. That is, as a first bifurcated half of the gate is moved vertically in one direction, the gate width is expanded and when moved vertically in the opposite direction the gate width is also expanded. This design has a potentially serious problem with wedging action in the upward, that is the open position of the gate due to high opening torques. In contrast, the valve of this invention has an improved gate design to cause the gate to wedge only in the valve closed position and the gate to contract in width as the gate is moved to the open position to thereby substantially reduce opening torque.